The phase behavior of fully hydrated egg yolk phosphatidylcholine (EPC)/diolein (DO) mixtures was investigated by small-angle X-ray scattering and P-31 NMR. EPC formed a lamellar phase, whereas DO gave a negative curvature to the lipid membranes, leading to nonlamellar phase formation. At an EPC/DO molar ratio of 8:2, the mixture formed a bicontinuous cubic phase of primitive type. A bicontinuous cubic phase of diamond type was also observed at the ratio of 7:3, but it coexisted with an inverted hexagonal (H-II) phase. The mixture with higher DO fractions formed the H-II phase. We further investigated if these nonlamellar phases could be dispersed by high-pressure emulsification with Pluronic F127, to form cubosomes or hexosomes, as have been obtained in monoolein-based cubic and H-II phases (Langmuir 2001, 17, 39173922; Langmuir 2002, 18, 9283-9288). As a result, cubosomes could not be obtained and vesicles were formed instead, presumably due to the existence of F127, which gave a positive curvature to the negatively curved membranes. However, the H-II phase, which was less compatible with F127, could be dispersed to form hexosomes.